The realization of a data density of 1 Terabyte/inch2 (1 Tbit/in2) depends, in part, on designing a head-disk interface (HDI) with the smallest possible head-media spacing (“HMS”). Head-media spacing refer to the distance between a read or write sensor and a surface of a magnetic media. A discussion of head-media spacing is found in U.S. patent application Ser. No. 12/424,441, entitled Method and Apparatus for Reducing Head Media Spacing in a Disk Drive, filed Apr. 15, 2009, which is hereby incorporated by reference.
Read-write heads for disk drives are formed at the wafer level using a variety of deposition and photolithographic techniques. Multiple sliders, up to as many as 40,000, may be formed on one wafer. The wafer is then sliced into slider bars, each having up to 60-70 sliders. The slider bars are lapped to polish the surface that will eventually become the air bearing surface. A carbon overcoat is then applied to the slider bars. Finally, individual sliders are sliced from the bar and mounted on gimbal assemblies for use in disk drives.
Slider bars with trailing edges composed of metallic layers and ceramic layers present very severe challenges during lapping. Composite structures of hard and soft layers present differential lapping rates when lapped using conventional abrasive lapping plates. The variable polishing rates of the metallic and ceramic materials lead to severe recessions, sensor damage, and other problems.
Current lapping typically involves a tin plate charged with small diamonds with an average diameter of about 250 nm. The charging process embeds the diamonds into the soft tin material. The lapping plate is flooded with a lubricant (oil or water based). The viscosity of oil based lubricants is about 4 orders of magnitude greater than the viscosity of air. The lubricant causes a hydrodynamic film to be generated between the slider bar and the lapping plate. The hydrodynamic film is critical in establishing a stable interface during the lapping process and to reduce vibrations and chatter. To overcome the hydrodynamic film a relatively large force is exerted onto the slider bar to cause interference with the diamonds necessary to promote polishing. A preload of about 1 kg is not uncommon to engage a single slider bar with the lapping media.
The preload is typically determined by the density of the diamonds and the diamond height variation. As the industry moves to nano-diamonds smaller than 250 nm, the preload will need to be increased to overcome the fluid dynamic film. Nano-diamonds are difficult to embed in the tin plate. The risk of free diamonds damaging the slider bar increases. Precisely grooved plates or lubricant reformulation will be required to overcome the fluid dynamic film.
Variables such as lapping media speed, preload on the slider bar load, nominal diamond size, and lubricant type must be balanced to yield a desirable material removal rate and finish. A balance is also required between the hydrodynamic film and the height of the embedded diamonds to achieve an interference level between the slider bar and the diamonds.
FIG. 2 is a schematic side sectional view of a conventional slider bar including a plurality of individual sliders before lapping. Each slider in the slider bar typically includes read-write transducers. As used herein, “read-write transducer” refers to one or more of the return pole, the write pole, the read sensor, magnetic shields, and any other components that are spacing sensitive.
FIG. 3 illustrates the bar of FIG. 2 after lapping with a diamond-charged lapping plate. The diamond-charged plates cause large transducer protrusion and recession variations, contact detection area variation, substrate recession, microscopic substrate fractures leading to particle release during operation of the disk drive, scratches from free diamonds, and transducer damage.
A thicker carbon overcoat is often used to compensate for transducer recession and protrusion. Increasing the carbon overcoat, however, results in increased HMS and lower data densities. Transducer recession and protrusion also results in unpredictable transducer location leading to both disk drive reliability issues associated with lower slider clearance and yield issues associated with high slider clearance. Consequently, current lapping techniques result in lower yields and/or higher head media spacing, with a corresponding increase in cost and/or a decrease in data densities.
Meyer et al., Proximity Recording—The Concept of Self-Adjusting Fly Heights, Vol. 33, No. 1 IEEE Transactions On Magnetics p. 912 (1997) (hereinafter “Meyer”) disclosed a method of reducing head media spacing by reducing the clearance between the head and media to zero. FIG. 1 shows a slider designed to be in contact with a polishing media. The media used had a peak to peak roughness of about 25 nanometers (1×10−9 meters) with an amorphous carbon overcoat. The trailing edge of the slider was in contact with the disk texture with an interference level of 25 nm. The combination of media hardness and localized stresses at the trailing edge of the slider caused burnishing to occur. The polishing level and smoothness of Meyer is far superior to current lapping techniques.
U.S. Pat. Nos. 5,632,669 and 5,855,131 (Azarian et al.) discloses an interactive system for lapping transducers has an abrasive surface. The lapping body contains a magnetic medium layer that is either prerecorded or written by the head during lapping. The signal received by the head is monitored and analyzed by a processor in order to determine, in part, when to terminate lapping. A series of transducers can be simultaneously lapped while individually monitored, so that each transducer can be removed from the lapping body individually upon receipt of a signal indicating that transducer has been lapped an optimal amount. Azarian teaches continuous contact lapping, such as disclosed in Meyer. The individual heads are not gimbaled and the lapping is performed without water or other lubricants. No method is proposed in Azarian for applying a carbon overcoat to the individual heads after lapping.
Strom et al., Burnishing Heads In-Drive for Higher Density Recording, Vol. 40, No. 1 IEEE Transactions On Magnetics p. 345-348 (2004) and Singh et al., A Novel Wear-in-Pad Approach to Minimizing Spacing at the Head/Disk Interface, Vol. 40, No. 4 IEEE Transactions on Magnetics, p. 3148-3152 (2004) replicated the results from Meyer by flying an individual slider over a textured disk surface. An air bearing was established at the leading edge of the slider to provide stability during the burnishing process. An improvement was found in the surface finish between the diamond lapped surfaces (upper) and the burnish lapping under low interfacial forces (lower).
U.S. Pat. No. 7,367,875 (Slutz et al.) discloses a polishing pad conditioning head with a substrate, at least one ceramic material, at least one carbide-forming material, and a chemical vapor deposited diamond coating disposed on at least a portion of a surface of the substrate. The diamond grit has an average grain size ranging from about 1 to about 15 microns. As discussed above, the diamond abrasives are too aggressive to provide atomic level burnishing.
U.S. Pat. No. 7,189,333 (Henderson) discloses end effectors for conditioning planarizing pads. The end effector includes a first surface with a plurality of generally uniformly shaped contact elements. The contact elements can have a wear-resistant, carbon-like-diamond, silicon, and/or silicon carbide layer. The protrusions of Henderson are on the order of about 50 micrometers high.
U.S. Pat. No. 6,872,127 (Lin et al.) discloses conditioning pads used in the chemical mechanical polishing of semiconductor wafers. The conditioning pad includes multiple, pyramid-shaped, truncated protrusions which are cut or shaped in the surface of a typically stainless steel substrate. A seed layer, typically titanium nitride (TiN), is provided on the surface of the protrusions, and a contact layer such as diamond-like carbon (DLC) or other suitable film is provided over the seed layer. The protrusions of Lin are on the order of about 0.2 millimeters high. The patterned geometric features of Henderson and Lin rely on significant pressure to initiate material removal, which is inconsistent with atomic level material removal.
Various methods and systems for finish lapping read-write transducers are disclosed in U.S. Pat. No. 5,386,666 (Cole); U.S. Pat. No. 5,632,669 (Azarian et al.); U.S. Pat. No. 5,885,131 (Azarian et al.); U.S. Pat. No. 6,568,992 (Angelo et al.); and U.S. Pat. No. 6,857,937 Bajorek), which are hereby incorporated by reference.